2\ 0. BosKorth — Outlines of Oilfield Geology. 17 



use to-daj'. The writer therefore hopes that an orderly summary of 

 present knowledge may be useful. 



Pakt I. Theokexical Considerations. 



Oeigin. — Within a petroliferous formation as found to-day the oil is 

 concentrated in certain positions in the most porous beds, whilst often 

 the intervening rocks show little or no indication of oil. The origin 

 of the petroleum has been much discussed, especially in American, 

 Hussian, and German literature. 



The theories may be placed under two heads according to the 

 sujDposed source of the carbon. 



1. Inorganic Origin. — It is held that the oil has been introduced 

 into the rocks from deep-seated sources in or beneath the earth's 

 crust, where it was perhaps produced by chemical reactions between 

 such substances as water, carbonates, carbides, carbon dioxide, etc., 

 and some authorities maintain that the intrusion of hydrocarbons is 

 one of the phenomena of vulcanicity, similar to the intrusion of 

 igneous magma. For many reasons these theories have been rejected 

 by the majority of ' oil-geologists ', as they do not accord with the 

 distribution of the petroleum. 



2. Organic Origin.- — It is held that the hydrocarbons have been 

 produced by chemical changes in the organic matter present in the 

 sediments. The organic origin is generally accepted, though there 

 is much difference of opinion as to what animals or plants (terrestrial 

 and marine) can give rise to petroleum and under what conditions. 



The facts differ widely in different fields. In some the oil-rocks 

 occur amidst thousands of feet of strata rich in marine fossils. 

 In others they are amidst great thicknesses of shallow water, coastal 

 or estuarine deposits, in which both animal and vegetable remains 

 abound, e.g. Peru. Eich oil-sands are sometimes closely associated 

 with coal-seams and lignites in formations where the organic matter 

 apparently is mainly vegetable and terrestrial, e.g. in Assam. (Small 

 quantities of oil have occasionally been met with in various British 

 coal-fields.) ' Again, in some fields the oil horizons are unfossiliferous 

 sands separated and underlain by thousands of feet of clays and 

 shales, most of which are entirely devoid of visihle fossil remains, 

 e.g. Trinidad. 



In some fields the oil-rocks are associated with salt and gypsum, 

 bearing formations from which the oil seems always to have originated, 

 e.g. the Eoumanian Oilfields. Yet again the oil-rocks occur in 

 association with diatomaceous sediments. From such Dr. Half Arnold 

 informs me there can be no doubt that much of the Californian oil has 

 been derived. 



It is supposed that the oil begins to form from the organic matter 

 not long after deposition, as films and globules around and among the 

 rock particles, accompanied by the salt water of sedimentation ; but 

 nothing definite is known as to the conditions of temperature and 

 pressure necessary. 



■^ Commercial quantities of oil from the Scottish Oilfield are not found 

 present in the rocks as oil, but are produced on destructive distillation of 

 certain carbonaceous shales. 



DECADE V. — VOL. IX. — NO. I. 2 



